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1144 CHAPTER 52 1955 the thin bars has disappeared, since the small air jets have reunited and the?? air stream has not yet spread beyond the core dimensions. With deflecting! grilles the exploring jet should be turned to the angle giving a maximum!;! reading. With suitable traversing tips and calibration, this instrument's feS?' may also be used on exhaust grilles if proper grille factors are applied.8^ Those contemplating such measurements should consult the refere: cited. Smoke is a qualitative tool which is very useful in studying air move-;!; P ments. Satisfactory smoke can be obtained from titanium tetra-chloride- 7 (which, however, is very irritating to nasal membranes) or by mixing potassium chlorate and powdered sugar (a non-irritating smoke) and firing the mixture with a match. This latter process evolves considerable heat, and it should be confined in a pan away from flammable materials. The titanium tetra-chloride smoke lends itself to spot determinations, particu larly for leakage through casings and ducts, as it can be easily handled in a small pistol-like ejector. The fumes of aqua ammonia and of sulfuric acid, if permitted to mix, form a white precipitate which is useful for some purposes. Two bottles, one containing ammonia water and the other acid, are connected to a common nozzle by means of rubber tubing. Air is forced over the surfaces of the liquids in the bottles by means of a syringe, and the two streams, upon mixing at the nozzle, form a white cloud. -iff The Kata Thermometer The measurement of air current velocities within enclosed spaces, such as the rooms of a house, is usually a tedious undertaking. However, useful data can be obtained by using the instruments described in following para graphs if they are maintained in calibration, and the user understands the operation and limitations of the instruments.*2 One of the instruments useful for determining the velocity of air currents in free spaces is the Kata thermometer which is essentially an. alcohol thermometer with a large bulb. The instrument is heated above 100 F, and then the time in seconds required for it to cool from 100 to 95, when located in the air current, gives a measure of the air speed. It is important to have the Kata thermometer dry before taking the reading. Each Kata has its own factor etched on the stem, and this factor must be used with its cooling formula or chart for obtaining the velocity. The Kata thermom eter is useful in exploring ventilated spaces to determine whether the proper air movement and distribution are being maintained. It is also used m determining the cooling power of the atmosphere, since it loses heat by radiation and convection when dry, and by radiation, convection, and evaporation when the bulb is equipped with a wetted cloth covering.33 Thermal Anemometers If a suitable sensing element is heated electrically at a fixed rate and exposed to an air stream, the temperature difference between the element and the stream becomes a measure of velocity by calibration. In "je kot-wire anemometer, a very fine heated wire is employed as a resistancethermometer element whose temperature may be determined accu rately.*4 36 In the heated-bulb thermometer type, a heating wire is wound around the bulb of a mercury-in-glass thermometer, and the temperature difference between this thermometer and a similar unheated one serves as an index of air speed.** The heated-thermocouple anemometer is cali brated to give velocity in terms of the differential emf between heated an Instruments and Measurements 1145 unheated thermo-junctions exposed to an air stream.*4'37 Combined measurements of air temperature and velocity are particularly useful for air distribution studies, and automatic recording potentiometers or resist ance-thermometer devices facilitate spatial traverses. A correct calibra tion of thermal anemometers requires consideration of the effects of tem perature, humidity and pressure upon the air properties which influence convective heat transfer.*7 With sensing elements of simple, shapes for which convection data are known, thermal anemometers may be designed, both thermally and electrically, for desired characteristics. Directional sensitivity is - controllable. Thermal anemometers are convenient and practical for low velocities. Infiltration or Air Change Measurement The total air leakage into an existing building caused by wind and tem perature difference forces can be determined with considerable accuracy by determining the decrease in concentration of a tracer gas. Hydrogen and helium have both been used rather successfully as tracer gases in con junction with a sensitive thermal conductivity comparator.38 About one percent of tracer gas by volume is introduced into the room or building, and the tracer gas is thoroughly mixed with the air. The decrease in con centration of the tracer gas is observed at regular time intervals with a thermal conductivity meter as infiltration dilutes the mixture. The fol lowing formula is used to calculate the infiltration from the observed concentration measurements: C=C,,e~~ (7) where C = initial tracer gas concentration, percent. C = concentration of tracer gas after t minutes, percent. v = volume of room or building, cubic feet. k = infiltration rate, cubic feet per minute. e = 2,718, for natural logarithms. ^Closets and cupboards should be left open during such infiltration tests so the entire space will have the same rate of change of tracer-gas concen tration. HUMIDITY MEASUREMENT Psychrometers Any instrument capable of measuring the humidity or hygrometric state !u-r *s a hysrometer. A psychrometer is a particular kind of hygrome er which consists of two mercury thermometers, one of which has a cloth ''lel) r sock applied to its bulb. For use, the wick is wetted with water and ^ entilated with air moving at a recommended rate of 900 fpm or more, rela- th 6 -r lns^,rumenb.30 In the simpler and more common type, known as ,, ? slmg psychrometer, the two thermometers are mounted side by side on rame fitted with a handle by which the device can be whirled through jair` . ^he motion is arrested for reading the thermometers, and contin- the U t b *i*e ^ermometer readings become steady. Due to evaporation, bulh'fh U k thermometer will indicate a lower temperature than the drysion th!'raometer and the difference is known as the wet-bulb depres,l ' Charts and tables are available showing the relation between the totajnome*'er rea(fir'gs and the humidity.40'41 Data are usually based on a is n ^ressure one standard atmosphere. For precise work, a correction ecessary for barometric pressure and is usually made by multiplying